Organisms must synthesize nucleotides in order for their cells to divide and replicate. Nucleotide synthesis in mammals may be achieved through one of two pathways: the de novo synthesis pathway; or the salvage pathway. Different cell types use these pathways to differing extents.
Inosine-5′-monophosphate dehydrogenase (IMPDH; EC 1.1.1.205) is an enzyme involved in the biosynthesis of guanine nucleotides. IMPDH catalyzes the NAD-dependent oxidation of inosine-5′-monophosphate (IMP) to xanthosine-5′-monophosphate (XMP) [Jackson R. C. et al., Nature, 256, pp. 331-333, (1975)]. Regardless of species, the reaction involves the random addition of substrates. A conserved active site Cys residue attacks the C2 position of IMP and hydride is transferred to NAD, producing NADH and the E-XMP* intermediate. NADH is released and a mobile flap folds into the vacant NADH site, E-XMP* hydrolyzes and XMP is released [W. Wang and L. Hedstrom, Biochemistry 36, pp. 8479-8483 (1997); J. Digits and L. Hedstrom, Biochemistry 38, pp. 2295-2306 (1999); Gan et al., Biochemistry 42, pp 847-863 (2003)]. The hydrolysis step is at least partially rate-limiting in all of the IMPDHs examined to date. The enzyme is unusual in that a large conformational change occurs in the middle of a catalytic cycle.
IMPDH is ubiquitous in eukaryotes, bacteria and protozoa [Y. Natsumeda & S. F. Carr, Ann. N.Y. Acad., 696, pp. 88-93 (1993)]. The prokaryotic forms share 30-40% sequence identity with the human enzyme. Two isoforms of human IMPDH, designated type I and type II, have been identified and sequenced [F. R. Collart and E. Huberman, J. Biol. Chem., 263, pp. 15769-15772, (1988); Y. Natsumeda et al., J. Biol. Chem., 265, pp. 5292-5295, (1990)]. Each is 514 amino acids, and they share 84% sequence identity. Both IMPDH type I and type II form active tetramers in solution, with subunit molecular weights of 56 kDa [Y. Yamada et al., Biochemistry, 27, pp. 2737-2745 (1988)].
The de novo synthesis of guanine nucleotides, and thus the activity of IMPDH, is particularly important in B- and T-lymphocytes. These cells depend on the de novo, rather than salvage pathway to generate sufficient levels of nucleotides necessary to initiate a proliferative response to mitogen or antigen [A. C. Allison et al., Lancet II, 1179, (1975) and A. C. Allison et al., Ciba Found. Symp., 48, 207, (1977)]. Thus, IMPDH is an attractive target for selectively inhibiting the immune system without also inhibiting the proliferation of other cells.
Immunosuppression has been achieved by inhibiting a variety of enzymes including, for example, the phosphatase calcineurin (inhibited by cyclosporin and FK-506); dihydroorotate dehydrogenase, an enzyme involved in the biosynthesis of pyrimidines (inhibited by leflunomide and brequinar); the kinase FRAP (inhibited by rapamycin); and the heat shock protein hsp70 (inhibited by deoxyspergualin). [See B. D. Kahan, Immunological Reviews, 136, pp. 29-49 (1993); R. E. Morris, The Journal of Heart and Lung Transplantation, 12(6), pp. S275-S286 (1993)].
Inhibitors of IMPDH are also known. U.S. Pat. No. 5,380,879 (incorporated by reference) and U.S. Pat. No. 5,444,072 (incorporated by reference) and PCT publications WO 94/01105 and WO 94/12184 describe mycophenolic acid (MPA) and some of its derivatives as potent, uncompetitive, reversible inhibitors of human IMPDH type I (Ki=33 nM) and type II (Ki=9 nM). MPA has been demonstrated to block the response of B- and T-cells to mitogen or antigen [A. C. Allison et al., Ann. N.Y. Acad. Sci., 696, 63, (1993)].
Immunosuppressants, such as MPA, are useful drugs in the treatment of transplant rejection and autoimmune diseases. [R. E. Morris, Kidney Intl., 49, Suppl. 53, S-26, (1996)]. However, MPA is characterized by undesirable pharmacological properties, such as gastrointestinal toxicity and poor bioavailability. [L. M. Shaw, et al., Therapeutic Drug Monitoring, 17, pp. 690-699, (1995)].
A novel noncompetitive inhibitor of IMPDH, merimepodib, has immunosuppressive activity, is orally bioavailable, and inhibits the proliferation of primary human, mouse, rat, and dog lymphocytes at concentrations of ˜100 nM. Studies have demonstrated that merimepodib a potent, specific, and reversible IMPDH inhibitor that selectively inhibits lymphocyte proliferation. It is currently in clinical trials to treat hepatitis C virus.
Nucleoside analogs such as tiazofurin, ribavirin and mizoribine also inhibit IMPDH [L. Hedstrom, et al. Biochemistry, 29, pp. 849-854 (1990); L. Hedstrom, et al. Curr. Med. Chem. 1999, 6, 545-561]. These compounds require activation to either the adenine dinucleotide (tiazofurin) or monophosphate derivatives (ribavirin and mizoribine) that inhibit IMPDH. These activation pathways are often absent in the cell of interest. In addition, nucleoside analogs suffer from lack of selectivity and can be further metabolized to produce inhibitors of other enzymes. Therefore, nucleoside analogs are prone to toxic side effects.
Mycophenolate mofetil, a prodrug which quickly liberates free MPA in vivo, was recently approved to prevent acute renal allograft rejection following kidney transplantation. [L. M. Shaw, et al., Therapeutic Drug Monitoring, 17, pp. 690-699, (1995); H. W. Sollinger, Transplantation, 60, pp. 225-232 (1995)]. Several clinical observations, however, limit the therapeutic potential of this drug. [L. M. Shaw, et al., Therapeutic Drug Monitoring, 17, pp. 690-699, (1995)]. MPA is rapidly metabolized to the inactive glucuronide in vivo. [A. C. Allison and E. M. Eugui, Immunological Reviews, 136, pp. 5-28 (1993)]. The glucuronide then undergoes enterohepatic recycling causing accumulation of MPA in the gastrointestinal tract where it cannot exert its IMPDH inhibitory activity on the immune system. This fact effectively lowers the drug's in vivo potency, while increasing its undesirable gastrointestinal side effects.
IMPDH also plays a role in other physiological events. Increased IMPDH activity has been observed in rapidly proliferating human leukemic cell lines and other tumor cell lines, indicating IMPDH as a target for anti-cancer as well as immunosuppressive chemotherapy [M. Nagai et al., Cancer Res., 51, pp. 3886-3890, (1991)]. IMPDH has also been shown to play a role in the proliferation of smooth muscle cells, indicating that inhibitors of IMPDH, such as MPA, may be useful in preventing restenosis or other hyperproliferative vascular diseases [C. R. Gregory et al., Transplantation, 59, pp. 655-61 (1995); PCT publication WO 94/12184; and PCT publication WO 94/01105].
Additionally, IMPDH has been shown to play a role in viral replication in some viral cell lines. [S. F. Carr, J. Biol. Chem., 268, pp. 27286-27290 (1993)]. Analogous to lymphocyte and tumor cell lines, the implication is that the de novo, rather than the salvage, pathway is critical in the process of viral replication.
Cryptosporidiosis is a severe gastrointestinal disease caused by protozoan parasites of the genus Cryptosporidium. The most common causes of human disease are C. parvum and C. hominis, though disease can also result from C. felis, C. meleagridis, C. canis, and C. muris infection. Small children, pregnant women, the elderly, and immuno-compromised people (e.g., AIDS patients) are at risk of severe, chronic and often fatal infection. [Carey, C. M., Lee, H., and Trevors, J. T., Water Res., 38, 818-62 (2004); and Fayer, R., Veterinary Parasitology, 126, 37-56 (2004)]. The Cryptosporidium parasites produce spore-like oocysts that are highly resistant to water chlorination. Several large outbreaks in the U.S. have been linked to drinking and recreational water. Infection rates are extremely high, with disease manifest in 30% of exposed individuals and a 50-70% mortality rate among immuno-compromised individuals. Furthermore, there is a growing and credible concern that these organisms could be deliberately introduced into the water supply in an act of bioterrorism. Effective drugs are urgently needed for the management of cryptosporidiosis in AIDS patients and/or epidemic outbreaks.
All parasitic protozoa lack purine biosynthetic enzymes and must salvage purines from their hosts, making this pathway an extremely attractive target for developing anti-protozoal drugs. IMPDH is a key enzyme in the purine salvage pathway of C. parvum. As discussed above, IMPDH is a validated drug target in immunosuppressive, cancer and viral therapy, so the human enzymes are extremely well studied. It has recently been shown that C. parvum IMPDH has very different properties than the human enzymes and that IMPDH inhibitors block parasite proliferation in vivo [N. N. Umejiego et al., J Biol Chem, 279 pp. 40320-40327 (2004); and B. Striepen et al., Proc Natl Acad Sci USA, 101 pp. 3154-9 (2004)].
Thus, there exists a need for potent IMPDH inhibitors with improved pharmacological properties and selectivities. Such inhibitors should have therapeutic potential as immunosuppressants, anti-cancer agents, anti-vascular hyperproliferative agents, antiinflammatory agents, antifungal agents, antipsoriatic and anti-viral agents. Specifically, there is a need for selective IMPDH inhibitors that can slow or block parasite and bacterial proliferation. The present invention fulfills this need and has other related advantages.